The present disclosure generally relates to signal processing. In particular, a technique for adjusting a phase relationship among modulation symbols is presented.
In multi-transmitter communications networks, channel access techniques allow multiple transmitters connected to the same physical channel to share its transmission capacity. Various such channel access techniques are known in the art. For example, in second generation communications systems according to the Global System for Mobile communications (GSM) standard, Time Division Multiple Access (TDMA) techniques are utilized to divide a specific frequency channel into individual time slots assigned to individual transmitters. In third generation communications systems, Code Division Multiple Access (CDMA) techniques divide channel access in the signal space by employing a combination of spread spectrum operations and a special coding scheme in which each transmitter is assigned an individual code.
The next advance in wireless communications systems considers Orthogonal Frequency Division Multiple Access (OFDMA) techniques to achieve still higher bit rates. One major advantage of OFDMA techniques over other channel access techniques is their robustness in the presence of multi-path signal propagation. On the other hand, the waveform of OFDMA signals exhibits very pronounced envelope fluctuations resulting in a high Peak-to-Average Power Ratio (PAPR). Signals having a high PAPR require highly linear power amplifiers to avoid excessive inter-modulation distortion, and the power amplifiers have to be operated with a large back-off from their peak power. These demands result in a lower power efficiency, which places a significant burden specifically on battery operated transmitters as utilized in mobile telephones and similar portable user equipment. The high PAPR inherent to OFDMA techniques is to a certain extent overcome by Single Carrier Frequency Division Multiple Access (SC-FDMA) techniques.
The Third Generation Partnership Project (3GPP) is considering using both OFDMA and SC-FDMA in next generation communications systems currently standardized in the Long Term Evolution (LTE) project. According to Section 5 of the 3GPP Technical Specification (TS) 36.211 “Physical Channels and Modulation”, V8.7.0 of May 2009, SC-FDMA will be implemented in LTE user equipment for the uplink direction towards the access network. OFDMA, on the other hand, will be used in the downlink direction from the LTE access network towards the user equipment.
An exemplary realization of an SC-FDMA modulator stage 10 for LTE user equipment is schematically illustrated in FIG. 1 (PRIOR ART). The modulator stage 10 receives as input signal a multilevel sequence of complex-valued data symbols in one of several possible modulation formats such as Binary Phase Shift Keying (BPSK) or 16 level Quadrature Amplitude Modulation (16-QAM). The data symbols are received by the modulator stage 10 in blocks containing N symbols each.
Every block of N data symbols is initially subjected to an N-point Discrete Fourier Transform (DFT) in a DFT block 12. The DFT block 12 spreads the N data symbols over M frequency points or sub-carriers (N<M) to obtain a frequency domain representation of the N data symbols that is input to a mapping block 14. The mapping block 14 outputs a set of M complex-valued sub-carrier amplitudes. Exactly N of these amplitudes (corresponding to the M data symbols) will be non-zero, while the remaining amplitudes have been set to zero. The N subcarrier amplitudes output by the mapping block 14 are transformed by an Inverse Fast Fourier Transform (IFFT) block 16 back into a time domain signal comprising the resulting SC-FDMA modulation symbols. Except for an omission of the DFT block 12 used to spread the bits of the input symbols over the available subcarriers, an OFDMA modulator stage has a similar configuration as the SC-FDMA modulator stage 10 shown in FIG. 1. For this reason, SC-FDMA is sometimes also interpreted as DFT-spread OFDMA.
FIG. 2 (PRIOR ART) illustrates a transmitter stage comprising the SC-FDMA modulator stage 10 of FIG. 1 (PRIOR ART). As generally shown in FIG. 2 (PRIOR ART), the modulation symbols output by the modulator stage 10 are subjected to further signal processing steps before being up-converted to Radio Frequency (RF) and transmitted.
In a first processing step, a Cyclic Prefix (CP) is inserted into each SC-FDMA modulation symbol. The CP provides a guard-time between two sequentially transmitted symbols or symbol blocks to reduce mutual interference caused by multi-path propagation. After the CP has been added, a phase rotation is individually applied to each SC-FDMA modulation symbol. The phase rotation impresses a frequency shift on the modulation symbols that amounts to half the distance between two adjacent sub-carriers, i.e. to 7.5 kHz. This frequency shift reduces interference by the direct current transmitter offset. After the phase rotation operation, the modulation symbols are properly scaled for digital-to-analog signal conversion before being up-converted to RF, amplified and transmitted via one or more antennas.
In the signal processing scenario illustrated in FIG. 2 (PRIOR ART), it would generally be possible to impress the frequency shift in the frequency domain by the SC-TDMA modulator stage 10 when performing the sub-carrier mapping. However, in the specific configuration of the SC-FDMA modulator stage 10, only integer multiples of the sub-carrier distance can be impressed given an IFFT size of N. The LTE specifications define a frequency shift amounting to a fraction of the sub-carrier distance. The frequency shifting operation in the transmitter stage 18 is therefore carried out downstream of the modulator stage 10 in the time domain using a phase rotation (as increasing the IFFT size is computationally expensive).
In an effort to simplify the frequency shifting operation in the time domain, one may think of impressing the frequency shift during up-conversion by tuning the local oscillator to fRF+7.5 kHz. Such an approach introduces a continuous phase ramp over a sequence of modulation symbols as shown in the upper portion of FIG. 3 (FIG. 3 illustrates the phase relationship among a sequence of four modulation symbols). The LTE specifications, however, define a symbol-wise phase ramp that crosses zero after the CP of each modulation symbol (see lower portion of FIG. 3). Accordingly, impressing a continuous phase ramp over multiple modulation symbols would corrupt the phase relationship among the modulation symbols.
EP 2 091 194 A1 teaches an SC-FDMA modulator stage comprising one or multiple phase rotation blocks. In one implementation, a phase rotation block is located downstream of an FFT/IFFT combination.